FIG. 12 illustrates a circuit diagram of a background direct-current converter having a boost chopper and a step-down converter of a half wave current resonance-type. In the direct-current converter shown in FIG. 12, the boost chopper has a configuration where a series circuit of a reactor L and a switching element Q3 such as MOSFET is connected to both ends of a direct-current power supply Vi, and a series circuit of a diode D3 and a capacitor Ci is connected between a drain and a source of the switching element Q3, boosts a voltage of the direct-current power supply Vi and supplies the boosted voltage to the step-down converter.
The step-down converter has a configuration where a series circuit of switching element Q1 such as MOSFET and a switching element Q2 such as MOSFET is connected to both ends of a capacitor Ci, a series circuit of a current resonance capacitor Cri, a primary winding N1 of a transformer T and a reactor Lr connected between a drain and a source of the switching element Q1, and a series circuit of a diode RC and a capacitor Co is connected to both ends of a secondary winding N2 of the transformer T and outputs a direct-current voltage, which is insulated by stepping down a voltage of the capacitor Ci, to the capacitor Co.
In the background art, even when an input voltage is largely varied, it is possible to obtain a stable output voltage by the configuration of the boost chopper and the step-down converter. Meanwhile, JP-A-2010-81736 discloses an example of the configuration of the boost chopper and the step-down converter, as the prior art technology.
Also, JP-A-11-262263 discloses that the configuration of the boost chopper, and the step-down converter is made by a half bridge converter configuration, as one converter.